Design of low voltage and high current DC-DC converter

1. Introduction

A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of the switch transistor to turn on and off to maintain a stable output voltage. Since the 1990s, switching power supplies have been introduced into various electronic and electrical equipment fields. Computers, programmable switches, communications, electronic testing equipment power supplies, control equipment power supplies, etc. have all widely used switching power supplies. With the development of power supply technology, low-voltage, high-current switching power supplies are increasingly valued because of their high technical content and wide application. Among switching power supplies, forward and flyback types have the advantages of simple circuit topology and input and output electrical isolation, and are widely used in small and medium power power conversion occasions. Compared with forward and flyback types, push-pull converters have high transformer utilization and large output power, and because of the use of MOS tubes, there is basically no excitation imbalance. Therefore, it is generally believed that push-pull converters are suitable for low-voltage, high-current, and high-power occasions.

2. Basic push-pull conversion technology

The circuit structure of the push-pull DC-DC converter is shown in Figure 1 (a), and the waveform is shown in Figure 1 (b). The push-pull inverter converts the DC voltage into an AC square wave and adds it to the primary side of the high-frequency transformer. There is only one diode voltage drop on the secondary side of the isolation transformer. When the switch tube 1 S is turned on, the diode 1 D is subjected to positive voltage and is turned on, while 2 D is cut off due to reverse bias; therefore,

3 Circuit Design

3.1 Design of the main circuit

The main circuit topology of the switching power supply is shown in Figure 2. The detailed parameters are as follows: the input voltage is 12 (1 ± 10%) V, the output voltage is 24 V, the output current is 12 A, and the operating frequency is 33 kHz. The main circuit adopts a push-pull circuit, and the main switch tube is IRFP064N. There are two 1000uF/50V parallel input filter capacitors at the input end of the main circuit, and a 2.2uH input filter inductor is connected to the positive terminal of the input circuit (the inductance value is the same as the output filter inductor). The design of the transformer in the circuit is similar to that of the transformer used in general converters. You only need to pay attention to the winding method and copper wire selection. Since the current of this converter is too large, multiple strands of fine wires are wound in parallel.

The synchronous rectification technology is used at the output end. In low-voltage and high-current power converters, if traditional ordinary diodes or Schottky diodes are used for rectification, due to their large forward conduction voltage drop (the forward voltage drop of low-voltage silicon diodes is about 0.7V, the forward voltage drop of Schottky diodes is about 0.45V, and the new low-voltage Schottky diodes can reach 0.32V), the rectification loss becomes the main loss of the converter, which cannot meet the needs of high efficiency and small size of low-voltage and high-current switching power supplies. The volt-ampere characteristic of MOSFET when it is turned on is a linear resistance, called the on-state resistance RDS. The on-state resistance of new low-voltage MOSFET devices is very small, such as: IRF2807 (75V, 82A) and IRL2910 (100V, 55A) are 0.013Ω and 0.026Ω respectively. When they pass 20A current, the on-state voltage drop is less than 0.2V. In addition, the power MOSFET has a short switching time and high input impedance. These characteristics make MOSFET the preferred rectifier device for low-voltage and high-current power converters.

The silicon oxide layer between the gate and source of MOSFET has a limited withstand voltage. Once it is broken down, it will be permanently damaged. Therefore, the maximum gate-source voltage is actually between 50-75V. If the voltage exceeds 75V, a voltage regulator should be connected to the gate. Considering the cost, IRF2807 is selected. It should be pointed out that the connection method of the MOS tube as a rectifier in the figure is wrong. Some readers may think that the connection method is wrong. This is because ordinary reference books do not describe the positive gate voltage reverse output characteristics of power MOSFET. In fact, in addition to the introduction of the unsaturated region, saturated region and cut-off region, the reverse resistance region should also be considered for power MOSFET. The reverse resistance region has similar channel characteristics to the forward resistance region. This is because the voltage on the secondary side of the transformer is an alternating square wave. The rectifier tube must withstand reverse pressure, but the power MOSFET is a reverse-conducting device. If it works in the forward resistance region, it will not be able to rectify.

In the voltage output part, an LC filter circuit is used. The inductance and capacitance parameters are calculated based on the K-type filter filter characteristic curve and calculation formula in LC filtering, and are adjusted after the experiment. (K-type filtering means that the product of the series arm impedance and the parallel arm impedance is a constant that does not change with frequency, and the dimension is resistance)

3.2 Design of control circuit

The control circuit uses the SG3525 chip, which is a current-controlled PWM controller for driving N-channel power MOSFET launched by Silicon General Semiconductor Company of the United States. It has high temperature stability and low noise level, undervoltage protection and external blocking functions, can easily achieve overvoltage and overcurrent protection, and can output two PWM signals with consistent waveforms and a phase difference of 180°, effectively reducing the ripple of the output current, and is suitable for push-pull topology circuits. The two control signals from the control chip SG3525 are used to control one IRFP064N respectively, achieving the purpose of driving two switch tubes, and the current directions of the two are opposite.

The control circuit uses a closed-loop control method to keep the output voltage constant. The detected voltage is transmitted to SG3525 after being isolated by an optical coupler and compared with a standard value to adjust the duty cycle and the output voltage accordingly, as shown in Figure 3. The detection of feedback voltage uses the 7840 optical coupler, which not only plays an isolation role but also makes the output voltage proportional to the input. Since the power supply required by the chip cannot be directly provided by the input power supply, two small DC voltage regulator chips are used to provide power. The reference source requires a stable voltage. Based on the voltage regulation output provided by the SG3525 itself, a TL431 voltage regulator is used to fully meet the requirements after measurement.

In the output rectifier circuit, when the rectifier tube Q3 is turned on by the forward voltage, Q3 should be driven to turn on in time to reduce the voltage drop and loss.

4 Experimental results and waveform analysis

Figure 5 is the waveform of two gate pulses in the push-pull circuit (oscilloscope amplitude * 10). The two pulses are basically symmetrical to each other. If the directions are opposite, the excitation directions are opposite to avoid excitation imbalance. The circuit works at about Vi = 11V at this time. Figure 5 is the output voltage of the transformer, which is the drive signal of the synchronous rectifier tubes Q3 and Q4. It can be seen from the figure that the upper and lower waveforms are symmetrical, indicating that they are only turned on when they are positive. The waveform of the output voltage was measured with an oscilloscope in the laboratory. The ripple is not large and can fully meet the requirements of electrical power supplies. The waveform obtained in the experiment is basically consistent with the analyzed waveform, except that at the moment of switch conversion, the voltage has a small spike, which is caused by the stray parameters of the circuit. The working efficiency of the circuit has been measured to be about 90%, which basically meets the design requirements.

5 Conclusion

The simulation analysis and experimental results verify the correctness of the theoretical analysis and formula derivation, indicating that the push-pull forward circuit applied to the converter has the following advantages:

1) The drain-source voltage spike of the switch tube is suppressed, the voltage stress and power loss of the switch tube are reduced, and the overall efficiency is high.

2) The transformer has bidirectional magnetization and high core utilization.

3) The input current ripple ampere-second integral is smaller than other topologies, reducing the size of the input filter.

4) After the output is filtered by LC, the output waveform amplitude is very small.

The author's innovation is: using push-pull technology to make the converter transformer utilization rate high and the output power large, and because of the use of MOS tubes, there is basically no excitation imbalance phenomenon. Synchronous rectification technology is used in the output part to reduce the voltage loss on the rectifier tube and improve the efficiency of the entire converter. LC filtering is used to basically eliminate the pollution of high-order waves.